It is well known that tabular silver halide grains having a high aspect ratio have high covering power, i.e., provide a high developed silver concentration per unit silver coverage, and there has been a demand for tabular grains having a further increased aspect ratio.
On the other hand, silver halide photographic materials containing a silver halide emulsion having a high silver chloride content have been in demand for achieving rapid processing and low-throughout replenishment.
Processes for preparing tabular grains having {100} faces as major faces (hereinafter referred to as {100} tabular grains) are described in detail in JP-A-5-204073 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-6-5936 and U.S. Pat. Nos. 4,063,951, 4,386,156, 5,275,930, 5,264,337, and 5,292,632. In these publications we can find, in particular, processes for producing {100} tabular silver chlorobromide grains, in which {100} tabular nuclei are formed by double jet addition of silver ions and chloride ions (with or without a trace of iodide ions) in the presence of chloride ions and a trace of iodide ions, or processes in which AgCl crystallite nuclei are previously formed and immediately thereafter silver ions and bromide ions (with or without chloride ions) are added to introduce a gap of halogen composition (halogen gap) thereby to form {100} tabular nuclei showing anisotropic growth.
The above-mentioned nucleus formation is generally followed by physical ripening and further growth. The growth step is often conducted at an elevated temperature to accelerate the growth.
The inventors of the present invention found the following very important for achieving a high aspect ratio and a monodispersion in the formation of silver chlorobromide {100} tabular grains:
(1) Between introduction of a halogen gap (formation of halogen nuclei of different halogen composition) and before the subsequent physical ripening-growth step, a step of depositing a silver halide phase which is more easily soluble than the phase so far built up is inserted to stabilize the {100} tabular nuclei. As a result, a large number of uniform nuclei can be obtained. That is, a nucleation step includes this step. PA1 (2) Anisotropic growth is accelerated under a low supersaturation condition. To this effect, it is desirable to conduct crystal growth at as high a temperature as possible. PA1 (3) While anisotropic growth is accelerated in the presence of fine grains, the anisotropic growth would be impaired if the fine grains have a wide size distribution. PA1 (1) A process for producing a silver halide emulsion comprising tabular silver halide grains containing not less than 10 mol% of silver chloride and having {100} faces as major faces and an average aspect ratio of 2 or higher, which tabular silver halide grains are prepared by comprising rising temperature of a grain formation system after completion of (a) nucleation step and taking (c) grain growth step, which process comprises adding silver ions in an amount corresponding to at least 1 mol% of the total silver content to the silver halide emulsion at a stage after introduction of a halogen gap in the nucleation step (a) and before the grain growth step (c). PA1 (2) The process for producing the silver halide emulsion according to (1), wherein the addition of silver ions is conducted within 10 minutes after introduction of a halogen gap. PA1 (3) The process for producing the silver halide emulsion according to (1), wherein the grain growth step (c) is carried out at a temperature higher than that of the nucleation step (a) by at least 20.degree. C. PA1 (4) A process for producing a silver halide emulsion comprising tabular silver halide grains containing not less than 10 mol% of silver chloride and having {100} faces as major faces and an average aspect ratio of 2 or higher, which tabular silver halide grains are prepared by comprising rising temperature of a grain formation system after completion of (a) nucleation step and taking (c) grain growth step, which process comprises allowing {100} tabular grains after completion of the nucleation step (a) to grow in the presence of a fine-grain emulsion having a coefficient of grain size variation of not more than 15%. PA1 (5) The process for producing the silver halide emulsion according to (1), wherein silver halide grains are allowed to grow by addition of fine grains at least 90% of which are capable of disappearance, with not less than 50% by number of all the added fine grains, when counted in the descending order of volume, being those having a volume falling within a range of 70 to 100% of the maximum volume of the grains capable of disappearance during the grain growth. PA1 (6) The process for producing the silver halide emulsion according to (1), wherein grain growth after addition of 30% of the total silver content is carried out by adding a silver salt and a halogen salt under conditions of pCl of 1.6 or more and a temperature of 65.degree. C. or higher at such a rate of addition that new nuclei may be formed and that the new nuclei may not grow to such a size that could not disappear ultimately. PA1 (7) The process for producing the silver halide emulsion according to (1), wherein pure silver chloride grains or silver halide grains containing not less than 10 mol% of chlorine having a coefficient of volume variation of not more than 0.2 and each having a volume of not greater than 0.001 .mu.m.sup.3 are used as seed crystals and said tabular grains have a coefficient of variation of not more than 0.25. PA1 (8) The process for producing the silver halide emulsion according to (6), wherein not less than 70% by number of all the added fine grains, when counted in the descending order of volume, are those having a volume falling within a range of 70 to 100% of the maximum volume of the fine grains capable of disappearance during the grain growth. PA1 (9) The process for producing the silver halide emulsion according to (1), wherein said tabular silver halide grains contain 20 to 99 mol% of silver chloride. PA1 (10) The process for producing the silver halide emulsion according to (7), wherein said seed crystals have a coefficient of volume variation of not more than 0.1. PA1 (11) The process for producing the silver halide emulsion according to (1), wherein (b) ripening step is carried out after the rising temperature and before the grain growth step (c). PA1 (12) A silver halide photographic material comprising a support having thereon at least one layer containing a silver halide emulsion comprising tabular silver halide grains containing not less than 10 mol% of silver chloride and having {100} faces as major faces and an average aspect ratio of 2 or higher, which tabular silver halide grains are prepared by comprising rising temperature of a grain formation system after completion of (a) nucleation step and taking (c) grain growth step, and further comprising adding silver ions in an amount corresponding to at least 1 mol% of the total silver content to the grain formation system at a stage after introduction of a halogen gap in the nucleation step (a) and before the grain growth step (c). PA1 (13) A silver halide photographic material comprising a support having on each side thereof a layer containing a silver halide emulsion comprising tabular silver halide grains containing not less than 10 mol% of silver chloride and having {100} faces as major faces and an average aspect ratio of 2 or higher, which tabular silver halide grains are prepared by comprising rising temperature of a grain formation system after completion of (a) nucleation step and taking (c) grain growth step, and further comprising adding silver ions in an amount corresponding to at least 1 mol% of the total silver content to the grain formation system at a stage after introduction of a halogen gap in the nucleation step (a) and before the grain growth step (c).
While JP-A-5-204073, JP-A-6-5936, and U.S. Pat. Nos. 5,275,930, 5,264,337, and 5,292,632 relate to {100} tabular grains having a high silver chloride content, little was it expected that deposition of silver halide prior to ripening would be of importance for monodispersion and stabilization of {100} tabular nuclei and achievement of a high aspect ratio of {100} tabular nuclei.
Neither was it expected to be important that the deposition be conducted within 10 minutes after formation of nuclei having a halogen gap, i.e., before ripening proceeds.
Further, although a mention of grain growth by addition of fine grains is given in JP-A-6-5936, importance of the degree of monodispersion of the fine grains for acceleration of anisotropic growth or monodispersion of silver chlorobromide {100} tabular grains was far beyond anticipation.